Nitrogen dioxide, according to the EPA, is a main ingredient in ground-level ozone, which can trigger serious respiratory problems. It also has also been shown to contribute to acid rain, global warming, deteriorations in water quality and "visibility impairment most noticeable in national parks," the EPA said.

Sulfur oxide has three common forms whichthat can contribute to respiratory illnesses and acid rain. It also has also been found to cause visibility impairment.

Both nitrogen dioxide and sulfur dioxide can be transported long distances, so the study will attempt to define what portion may be coming from local sources.

VOCs are any one of several organic vaporized chemical compounds that can react with other pollutants to form smog.

PM comes in two forms, based on size. According to the EPA, the smaller particles "pose the greatest problems, because they can get deep into your lungs, and some may even get into your bloodstream." Larger PM includes dust from roadways and industrial activities, and while it can be inhaled, it is much larger than the particles found in smog and generally poses a lower health risk. PM has been linked with several respiratory problems such as asthma and chronic bronchitis.

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